I have a '75 GMC, and found that the grills and mounting points did change over the years. I found a decent GMC grill from about a 1980 truck and decided to use it, but it mounted differently. I had to pull the left and right side mounting brackets (maybe the middle one too) from the donor truck so that I had brackets with holes in the right places. It would not fit on the original '75 vertical brackets.
I would love to find an OEM '75 GMC grill, since that year they included the engine size emblem on the lower driver's side of the grill. In later years they got rid of the engine size. I ran across an original grill at the Carlisle truck nationals one year, but didn't have a good way to ship it home. I should have figured out how to get it, since nobody makes a true reproduction '75 grill. GMC grills are much harder to find than Chevy grills, and you can't just swap logos between them.
You can look at the eL eM Cee truck catalog online and figure out what years are the same as your '75, since they group them by years. It looks like 75-78 should be the same. Your truck does not have an inside hood release, so universal grills usually have to be modified to provide a cutout for the exterior release handle.
Bruce